Electric vacuum switch



Sept. 4, 1945. w. 1. JONES, JR I 2,383,973

ELECTRIC VACUUM SWITCH Filed March 27, 1943 I ALKYD RES/IV COAT/N6 ALKXDRES/IV ca/m/ve Inventor: Wil I Lam L Jones J11,

b WW 6 His Attorney;

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Patented Sept. 4, 1945 2,383,973 ELECTRIC VACUUM SWlTCH William L.Jones, Jr., Schenectady, N. Y., assignor to General Electric Company, acorporation of New York application March 27, 194:3, Serial No. 480,745

2 Claims.

My invention relates to electric vacuum switches. and more particularlyto vacuum switches of the flexible diaphragm type for controlling highvoltage, high frequency circuits, such as control circuits associatedwith radio apparatus.

. craft has been proposed. However, in the practical application ofswitches to such circuits, a number of serious difficulties areencountered that may prevent proper and reliable operation of theswitch. For example, in aircraft operating at high altitudes the problemof voltage flash-over at the switch becomes very serious since a switchrated at a certain voltage at sea level will flash over at a muchreduced voltage at 40,000 feet Also, when operating at sea level insurface craft the switch must be capable of withstanding long exposureto the corrosive effects of salt water spray In addition to the aboveconsiderations, it is necessary that the switch be capable of thousandsof operations without impairment of the vacuum seal at the flexiblediaphragm associated with the movable contact. This is often a seriousproblem since a thin metal diaphragm sensitive to a small operatingforce may be liable to leakage which would destroy the vacuum within theswitch and cause failure thereof. On the other hand, a thick metaldiaphragm may be too stiff and insensitive to the operating force, andalso might crack under repeated use.

The principal object of my invention therefore is the provision of animproved vacuum switch of the aforesaid type that is particularlyadapted for high voltage, high frequency circuits at all flyingaltitudes and under practically all sea level conditions, that iscapable of operating efficiently a large number of times without failureboth under normal conditions and when subjected to corrosive agents suchas sea water spray, that is re-.

embodying the present invention and Fig. 2 is an elevational view,partly in section of the vacuum switch in a circuit closing positionthereof.

The single pole, double throw vacuum switch illustrated by way ofexample comprises an insulating glass envelope 1 having a flexible metalwall portion 2 comprising a diaphragm by which motion is transmittedfrom the exterior to the interior of the switch casing. As illustrated,the switch envelope or casing is T-shaped so thattwo laterally extendingportions la and lb of the glass envelope provide insulating mountingsfor the fixed lead-in terminals 3 and 4 which are preferably composed ofan iron, nickel and cobalt alloy. These terminals are provided withretrant seals 5 and 6, respectively, extending ma the switch casing inorder more effectively to insulate the terminals at high voltage. Withthis construction there is not only less metal within the envelope to bedegassed during the switch conditioning process, but the aforesaid alloyleads, which have high resistance at high frequencies, can be plated forthe most part with a lower resistance metal, such as cadmium, therebyreduc ing heating and permitting the switch'to be operated at. higherfrequencies. The terminals 3 and 4 extend to within a short distance ofeach other within the switch casing and terminate in the fixed contactsI and 8, which are composed of a suitable contact material such asmolybdenum.

The movable contact for grounding or connecting electrical apparatus toalternate circuits such as receiving and transmitting radio circuits,associated with the terminals 3 and 4 comprises a spade-like member 9composed of molybdenum for example, suitably attached at ill to theslotted free end of a tubular switch arm II. This arm is composed ofcopper-plated steel tubing and extends through and is copper brazed andsealed to the diaphragm 2. The contact arm II and diaphragm areelectrically connected through suitable connections exteriorly of theswitch to any apparatus or ground that is to be connected alternativelywith circuits connected to the contacts I and 8. -A suitable operatingmechanism (not shown) is adapted to tilt the movable contact arm H sothat the movable contact 0 engages either the upper contact I or thelower contact 8 or is in an intermediate position.

The metal wall or diaphragm 2 is united to the glass envelope I througha metal thimble l2 composed of a suitable metal, such as an iron, nickeland cobalt alloy whose coefficient of expansion is similar to that ofglass. The union of the thimble l2 to the glass envelope I is indicatedat I3 and the metal seal between the diaphragm 2 and the thimble isindicated at I4. The diaphragm 2 is suitably united to the thimble I2 bymeans of an overlapping circumferential flange 2a. This thimble, whichis electrically connected through the diaphragm to the movable contact,may conveniently form one terminal of the switch.

When the switch has been properly assembled and treated, it is evacuatedto a high degree through the tubular switch arm II, after which thetubular arm is flattened and sealed exteriorly of the casing at I I a.The technique of sealing and joining metal parts of the switch to glassparts, as well as the technique of degassing all interior parts of theswitch and evacuation of the switch casing are known to the art and arenot my invention. Therefore a description thereof is believed to beunnecessary for a complete understanding of the present invention.

The thickness of the diaphragm 2, which is composed of an iron, nickeland cobalt alloy above referred to, is exaggerated in the drawing, theactual thickness being approximately .003 inch. It will therefore :bseen that adequate protection of the sensitive diaphragm presents adifllcult problem from the standpoint of leakage due to mechanical wear,and to the effects of corrosion. It was found that additional metalplating not only added undesirable stiffness to the diaphragm butinvolved danger offgassing the tube interior during the plating process.

In accordance with my invention the diaphragm is coated exteriorly withan alkyd resin, specifically a rosin-modified, oil-modified, glycerylphthalate resin that is suitably thinned by a solvent, such as acoal-tar hydrocarbon, e. g. benzene or toluene. This alkyd resin coatingis indicated at and covers the entire exterior side of the diaphragm andextends into contact with the thimble ii to overlap the diaphragm flangeat 2a and also extends continuously from the diaphragm along the outersection of the switch arm i i. Since the alkyd resin is a good insulatorthe switch arm ii and its associated terminal can thereby be efficientlyinsulated with respect to the switch actuating mechanism.

I have found that this alkyd resin coating for the diaphragm is not onlya perfect seal for the thin diaphragm but also because of itsflexibility does not add appreciably to the stiflness of the diaphragm.Furthermore, the resin coating does not limit the normal long life ofthe diaphragm since it is capable of being flexed practicallyindefinitely without cracking. I have also found that this alkyd resincoating prevents the formation of corona at the diaphragm terminal whenthe switch is operating at high voltages. This prevention of corona ismost important at high altitudes where excessive corona discharge maycause fiashover and failure of the switch. I have further found thatthis alkyd resin coating is highly resistant to the corrosive influenceof sea water, and in fact my improved vacuum switch has withstoodwithout damage'a ZOO-hour, 20% salt spray test according to Governmentspecifications.

It will therefore be seen that I have solved in a most simple manner anumber of serious problems involved in the successful operation undersevere conditions of a high voltage vacuum switch of the flexiblediaphragm type.

The lead-in terminals 3 and 4 comprise conductor studs that are threadedat the outer ends as indicated for suitable connection with the circuitconductors. For the purpose of protecting the terminals from corrosion,the threaded portion of each stud is cadmium plated and the remainingportion, at 3a for example, between the threaded portion and the glassseal is also plated as previously described and in addition is coatedwith the alkyd resin above referred to. Therefore, the terminal as faras the threaded portion is both efllciently insulated and protected fromcorrosion. I have found that this additional insulation between theglass seal and the threaded portion is of material value in preventingcorona discharge. In addltion to the aforesaid advantages of myinvention, the alkyd resin coating is also resistant to the corrosiveefiects of oil.

It should be understood that my invention is not limited to specificdetails of construction and arrangement thereof herein illustrated, andthat changes and modifications may occur to one skilled in the artwithout departing from the spirit of my invention.

What I claim as new and desire to secure by Letters Patent 01' theUnited States is:

1. A vacuum switch for high voltage, highfrequency circuits comprisingan evacuated glass casing, a thin metal diaphragm composed of an iron,nickel and cobalt alloy forming a flexible wall portion of said casing,circuit controlling contacts separable within said casing, meansconnecting one of said contacts to said diaphragm whereby said contactcan be operated by means exterior to said casing, structure composed ofthe aforesaid alloy for joining said diaphragm to said glass casing, anda protective and insulating coating comprising an alkyd resin extendingcontinuously over the exterior of said diaphragm and to said joiningstructure.

2. A vacuum switch for high voltage, high frequency circuits comprisingan evacuated glass casing, a thin metal diaphragm of approximately .003inch thickness forming a flexible wall portion of said casing, circuitcontrolling contacts separable within said casing, means including atubular exhaust member connected to one of said contacts and extendingin sealed relation through said diaphragm whereby said contact can beoperated by means exterior to said casing, and a protective andinsulating coating completel covering the exterior wall of saiddiaphragm and extending continuously from said diaphragm over theexterior of said tubular exhaust member, said coating comprising analb'd resin.

WILLIAM L. JONES, Jn.

